Automatically expanding and contracting an antenna mounted on an automobile or any other vehicle involves expanding and contracting the antenna from inside the vehicle such as from inside the driver's room which, in practice, is very useful and has heretofore been widely employed. In automatically expanding and contracting the antenna, however, it is generally accepted practice to execute a clutch function when the antenna is fully expanded or is fully contracted to disconnect the drive force from the motor. So far, such a clutch mechanism has chiefly been incorporated in a portion of the rotary member such as a gear that drives the winding of the antenna expansion/contraction wire or that drives the operation wire. The clutch is put into operation when a rotational resistance (load) greater than a predetermined value is generated in such a portion.
Systems which do not employ clutch means have been disclosed in U.S. Pat. No. 4,153,825 and JP-AS No. 57-28962. In the mounted on a worm shaft that is driven by a motor being opposed thereto using a support ring, and a switch provided in a circuit for driving the motor in the forward direction or in the reverse direction is turned off by the operation member when an overload is exerted such as when the antenna is fully expanded or is fully contracted. If mentioned in detail, the worm and the operation member fastened to the worm are suppressed by a coil spring so as not to be thrusted in the axial direction in the load of the members at the back of the worm wheel when the antenna is expanded or contracted. When the antenna is fully expanded or is fully contracted, the operation is stopped after an overload is produced on the motor wheel. As the motor continues to rotate, therefore, the thrust that exceeds the suppressing force of the coil spring urges the worm such that an operation member is urged (one operation member is urged toward the coil spring interposed between the operation members at the time of forward rotation, and the other operation member is urged toward the coil spring at the time of reverse rotation), and whereby the switch is turned off to stop the motor.
In the latter system described in JP-AS No. 57-28962, on the other hand, a pair of circuit breakers are provided for a path that supplies electric power to turn in the forward direction the motor that drives a wire transfer mechanism which expands or contracts the antenna and for a path that supplies electric power to turn the motor in the reverse direction. When the antenna is fully expanded or is fully contracted, a heavy current flows into the power supply path through the circuit breaker; i.e., the power supply path is opened and the motor is stopped. The above circuit breakers are so coupled together that when a contact of one circuit breaker is opened, a reset button of the other circuit breaker is actuated. To realize such an operation, the two circuit breaker cases are coupled together as a unitary structure as shown in FIG. 2 of the above publication. A slidable reset button is provided between the breaker pieces that undergo thermal deformation when a heavy current flows in the cases, and a contact or a terminal of each of the drive power supply paths is provided on the breaker cases, so that the operation is repeated when the antenna is fully expanded or is fully contracted.
In the above-mentioned conventional systems, the clutch mechanism requires a considerably great clutch force; i.e., the clutch mechanism has a diameter which is so great as to be nearly equal to that of the drum. To obtain such a large clutch force, furthermore, a correspondingly large spring must be incorporated in the drum shaft. Therefore, the drum portion becomes considerably bulky in the axial direction, and the whole mechanism becomes bulky. Furthermore, in becomes difficult to maintain normal operation as the clutch force is weakened after the clutch operation is repeated many times or under poor local conditions.
In the above-mentioned conventional system disclosed in said U.S. Patent, furthermore, the switching operation or the switching force is obtained in the final output portion necessitating a large and rigid structure. That is, the operation for expanding or contracting the antenna finally requires an output which is greater than a predetermined value. To switch the clutch operation in the final output portion, furthermore, the members for clutch operation must inevitably be large in size and strong. In the above-mentioned general clutch, for example, the rotary member such as a drum or a gear that is a final output member has a large diameter so that it will not damage and will not deform a wire made of a synthetic resin. The clutch mechanism provided in the portion of the final output rotary member must exhibit braking action that meets the final output and, hence, must have a diameter comparable with that of the final output member. Furthermore, the spring employed therein must have a large resilient force; i.e., the spring becomes bulky. In the system of the above U.S. Patent, the worm must produce a considerably large output to drive the rotary wheel, and the operation member and the coil spring provided for the worm shaft via a support ring or the like must be made of a special material having considerably large strength and bulk. That is, the coil spring must effectively suppress the generation of thrust caused by the expanding or contracting force for the members at the back of the wheel that corresponds to the final output and must, hence, be considerably strong. The operation members are assembled employing such a strong coil spring, and are therefore bulky and rigid. The motor itself receives the load of the coil spring at all times, and directly acts on the final output portion, and must, hence, produce a large output. Furthermore, the motor shaft moves during the moment of overload and generates vibration which adversely affects the commutator. The overload directly acts even on the worm, and the teeth must have a sufficiently large strength.
In the system of JP-AS No. 57-28962, the operation is controlled electrically not mechanically, and a movable contact spring which is a circuit breaker must undergo deformation to overcome the action of a reversing spring when a heavy current flows therethrough to heat it. After the antenna is fully expanded or is fully contracted, therefore, a time lag is always involved before the circuit is opened by the circuit breaker and excessive force is always given to the mechanism during this moment. Therefore, the worm shaft, gear shaft and the mechanism for transferring the wire, that are driven by the motor, must nave considerably large strength. Moreover, the first and second circuit breakers provided in the drive power supply paths are coupled together, and a device having a slidable reset button is particularly provided between these circuit breakers so that one breaker is reset when the other one is opened. Therefore, the mechanism becomes complex and expensive, and generates heat during the ordinary operation which is not desirable.
When the device for automatically expanding and contracting the antenna is mounted on a vehicle, furthermore, there inevitably develop such abnormal conditions as bending of the antenna element, erroneous operation of the limit switch mechanism, drop in the voltage due to insufficiently charged condition of the car-mounted power source (storage battery), and external conditions due to change in environment. To cope with such abnormal conditions, it is necessary to provide a particular protection circuit (fuse or the like). It is further necessary to provide a timer circuit which secondarily shuts off the power source when an abnormal instruction signal is received from a sensor in the electronic circuit or when a current flows for longer than a predetermined period of current-carrying time, or a shut-off circuit employing a particular bimetal such as of the automatic reset type or the manual reset type.